1. Field of the Invention
The present invention relates to a method for producing a hydrogen permeation membrane, the hydrogen permeation membrane so produced, and the use of such a hydrogen permeation membrane.
2. Background Information
The peculiarity of palladium, as well as platinum to a considerably lesser extent, of enabling the rapid permeation of hydrogen, but of no other gas, although a membrane of this metal has long been known. In this way, hydrogen can be separated from other gases.
Hydrogen permeation cells that are used to produce hydrogen of the highest purity, have already been known for several decades. In these cells, a palladium alloy is typically used as the membrane. A construction made of tubules which are fixed on one end on a perforated plate (for instance by soldering) and are welded closed, but not fixed on the other has proved to be especially advantageous. The reason why this construction has proved itself especially advantageous in industry is that the palladium material, on absorbing hydrogen, expands, and thus if the tubules were fixed at both ends or if a plane foil were fastened in a rigid frame, would lead to a hydrogen-dictated expansion and thus to the occurrence of stresses and deformation in the palladium membrane, which would quickly cause tearing and porosity.
U.S. Pat. No. 5,215,729 discloses among other subjects a membrane for extracting hydrogen from hydrogen-containing fluid streams; it substantially comprises a first non-porous layer of a refractory metal and a second non-porous layer which is located directly on the first layer; the second layer substantially comprises palladium, palladium alloys, platinum, or platinum alloys. Tubes of small diameter are coated on the inside and outside with a suitable second layer by means of a special sputtering technique.
U.S. Pat. No. 3,350,846 discloses among other subjects a method of sputtering and vapor deposition of palladium onto niobium, vanadium and tantalum surfaces.
These methods known from the prior art can be employed, however, only for relatively large tubes (with an inside tube diameter of at least 25 mm).
It is also known that platinum can be precipitated out of a plurality of different baths either galvanically or currentlessly. Most often, baths on the basis of a palladium complex, palladium diaminodichloride (Pd(NH.sub.3).sub.2 Cl.sub.2) are used. With such baths, however, it is virtually impossible to coat the inside surface of tubes, especially tubes with a small inside diameter (maximum inside tube diameter of approximately 25 mm) of easily electroplatable materials. The problems reside both in the delivery of material from the electrolyte within the limited internal space in the tube and also the establishment of uniform electrical fields for the metal deposition in the tube. Furthermore, even under optimal conditions, the refractory metals are very difficult to coat using electroplating methods, since both in air and in contact with aqueous solutions, a thin, firmly adhering and nonconductive surface oxide film forms, which hinders the direct metal-to-metal contact between substrate and coating.
In physical deposition from the vapor phase by vapor deposition or sputtering, two problems arise. First, the vapor deposition material or the sputtering target must be positioned centrally in the tube, and from there must be applied to the inside of the tube by heating or by being bombarded with noble gas ions. Such methods are known for large tubes (inside tube diameter of approximately 100 mm), but not for smaller tubes. Second, the oxide barrier layer on the surface of the refractory metal (refractory metals or metals of the fourth and fifth secondary groups of the Periodic System of Elements) must be removed before the coating process. Normally, such layers are removed by "sputtering off", in which the oxide film is removed by being bombarded with noble gas ions. This process is not possible in the interior of a small tube (inside tube diameter less than 25 mm).
For the above reasons, it is not possible with methods known from the prior art to satisfactorily coat the inside of tubes of refractory metals having a small inside diameter (inside diameter of less that 25 mm) with palladium, a palladium alloy, platinum, or a platinum alloy.